Angiogenesis, defined as the growth of new capillary blood vessels or “neovascularization,” plays a fundamental role in growth and development. In mature humans, the ability to initiate angiogenesis is present in all tissues, but is held under strict control. A key regulator of angiogenesis is vascular endothelial growth factor (“VEGF”), also called vascular permeability factor (“VPF”). VEGF exists in at least four different alternative splice forms in humans (VEGF121, VEGF165, VEGF189 and VEGF206), all of which exert similar biological activities.
Angiogenesis is initiated when secreted VEGF binds to the Flt-1 and Flk-1/KDR receptors (also called VEGF receptor 1 and VEGF receptor 2), which are expressed on the surface of endothelial cells. Flt-1 and Flk-1/KDR are transmembrane protein tyrosine kinases, and binding of VEGF initiates a cell signal cascade resulting in the ultimate neovascularization in the surrounding tissue.
Aberrant angiogenesis, or the pathogenic growth of new blood vessels, is implicated in a number of conditions. Among these conditions are diabetic retinopathy, psoriasis, exudative or “wet” age-related macular degeneration (“ARMD”), rheumatoid arthritis and other inflammatory diseases, and most cancers. The diseased tissues or tumors associated with these conditions express abnormally high levels of VEGF, and show a high degree of vascularization or vascular permeability.
ARMD in particular is a clinically important angiogenic disease. This condition is characterized by choroidal neovascularization in one or both eyes in aging individuals, and is the major cause of blindness in industrialized countries.
A number of therapeutic strategies exist for inhibiting aberrant angiogenesis, which attempt to reduce the production or effect of VEGF. For example, anti-VEGF or VEGF receptor antibodies (Kim E S et al. (2002), PNAS USA 99: 11399–11404), and soluble VEGF “traps” which compete with endothelial cell receptors for VEGF binding (Holash J et al. (2002), PNAS USA 99: 11393–11398) have been developed. Classical VEGF “antisense” or aptamer therapies directed against VEGF gene expression have also been proposed (U.S. published application 2001/0021772 of Uhlmann et al.). However, the anti-angiogenic agents used in these therapies can produce only a stoichiometric reduction in VEGF or VEGF receptor, and the agents are typically overwhelmed by the abnormally high production of VEGF by the diseased tissue. The results achieved with available anti-angiogenic therapies have therefore been unsatisfactory.
RNA interference (hereinafter “RNAi”) is a method of post-transcriptional gene regulation that is conserved throughout many eukaryotic organisms. RNAi is induced by short (i.e., <30 nucleotide) double stranded RNA (“dsRNA”) molecules which are present in the cell (Fire A et al. (1998), Nature 391: 806–811). These short dsRNA molecules, called “short interfering RNA” or “siRNA,” cause the destruction of messenger RNAs (“mRNAs”) which share sequence homology with the siRNA to within one nucleotide resolution (Elbashir S M et al. (2001), Genes Dev, 15: 188–200). It is believed that the siRNA and the targeted mRNA bind to an “RNA-induced silencing complex” or “RISC”, which cleaves the targeted mRNA. The siRNA is apparently recycled much like a multiple-turnover enzyme, with 1 siRNA molecule capable of inducing cleavage of approximately 1000 mRNA molecules. siRNA-mediated RNAi degradation of an mRNA is therefore more effective than currently available technologies for inhibiting expression of a target gene.
Elbashir S M et al. (2001), supra, has shown that synthetic siRNA of 21 and 22 nucleotides in length, and which have short 3′ overhangs, are able to induce RNAi of target mRNA in a Drosophila cell lysate. Cultured mammalian cells also exhibit RNAi degradation with synthetic siRNA (Elbashir S M et al. (2001) Nature, 411: 494–498), and RNAi degradation induced by synthetic siRNA has recently been shown in living mice (McCaffrey A P et al. (2002), Nature, 418: 38–39; Xia H et al. (2002), Nat. Biotech. 20: 1006–1010). The therapeutic potential of siRNA-induced RNAi degradation has been demonstrated in several recent in vitro studies, including the siRNA-directed inhibition of HIV-1 infection (Novina C D et al. (2002), Nat. Med. 8: 681–686) and reduction of neurotoxic polyglutamine disease protein expression (Xia H et al. (2002), supra).
What is needed, therefore, are agents which selectively inhibit expression of VEGF or VEGF receptors in catalytic or sub-stoichiometric amounts.